The present invention relates to a process for treating steel-mill slag contained in a vessel in the liquid state, and to an apparatus for treating this slag.
In contrast to blast-furnace slag, which is used in the building sector after granulation and cooling, steel-mill slag obtained as a by-product when producing steel in a ladle or converter is employed as fertilizer because of its high content of phosphates as a result of the dephosphorisation of the steel. Whereas blast-furnace slag can be treated continuously as it is produced, steel-mill slag is produced intermittently in large quantities and, with the means available at the present time, cannot be cooled and granulated in the same way as blast-furnace slag.
Steel-mill slag, which floats on the surface of the bath of molten steel in a ladle or in a converter, is emptied into a vessel at the end of the steel treatment operation. It is subsequently conveyed, together with the vessel, to a dump or a storage site where the slag is cooled in block form. After being cooled in the open air, it is crushed to a fine powder which can be used as fertilizer. The powder is generally known as Thomas slag. To make it easier to use this slag, the powder can also be converted into granules.
This treatment of the cooled slag, particularly the crushing, generates an enormous amount of dust which is very difficult to control. In an effort to improve upon this process and because of the increasingly stringent environmental protection regulations, cooling and granulating the slag in water, as is done in the treatment of blast-furnace slag, has been tested. Unfortunately, these tests were unsuccessful because steel-mill slag is produced intermittently and is required to be stored in a vessel or ladle before its treatment. These tests therefore involved pouring the molten slag from the vessel by pouring the slag over the upper edge of the vessel and onto a granulation device in which the stream of molton slag undergoes the action of granulating water. These tests revealed several problems associated with pouring slag in this manner which prevented the process from being put into practice. It was impossible to prevent violent explosions attributable to the instantaneous evaporation of the water and the rapid changes of slag volume. This was exacerbated by the irregular pouring of the slag which, furthermore, can contain blocks of slag or even of steel in the pasty or solid state.